Reinforced shell mold and method

ABSTRACT

A method of making a ceramic shell mold comprises repeatedly coating a fugitive pattern of an article to be cast with a ceramic slurry layer and applying on the ceramic slurry layer a refractory stucco to form a plurality of ceramic slurry layers and stucco layers on the pattern wherein at least one of the stucco layers is formed by applying discontinuous stucco fibers followed by applying a granular stucco particles on the discontinuous stucco fibers.

FIELD OF THE INVENTION

[0001] The present invention relates to ceramic investment shell moldsfor casting molten metals and alloys and, more particularly, to ceramicshell molds that are fiber reinforced to improve mold strength at highcasting temperatures.

BACKGROUND OF THE INVENTION

[0002] Both the investment casting process and the lost wax shell moldbuilding process are well known, for example, as is apparent from theOperhall U.S. Pat. Nos. 3,196,506 and 2,961,751. The lost wax shell-moldbuilding process involves repeatedly dipping a wax or other fugitivepattern of the article to be cast in ceramic slurry to provide a ceramicslurry layer, draining excess slurry, stuccoing the slurry with coarseceramic particles to provide a stucco layer on the slurry layer, anddrying the layers to build up a shell mold of desired wall thickness onthe pattern. The green shell mold/pattern assembly then is subjected toa pattern removal operation to selectively remove the pattern from theshell mold. A commonly used wax pattern removal technique involves flashdewaxing where the green shell mold/pattern assembly is placed in anoven at elevated temperature to rapidly melt the wax pattern from thegreen shell mold. Following pattern removal, the green shell mold isfired at elevated temperature to develop mold strength for casting ofmolten metal or alloy therein.

[0003] Conventional lost wax ceramic shell molds can be prone to moldcracking or splitting during the pattern removal operation describedabove.

[0004] Attempts have been made to raise the capability of ceramic shellmolds in the DS casting of superalloy components. For example, U.S.Reissue Pat. No. 34,702 describes in one illustrative embodimentwrapping alumina-based or mullite-based reinforcement fiber in acontinuous spiral about an intermediate mold wall thickness as it isbeing built-up. U.S. Pat. No. 6,364,000 discloses in one illustrativeembodiment positioning one or more continuous carbon-based reinforcementfibers in a ceramic shell mold wall to this end.

SUMMARY OF THE INVENTION

[0005] The present invention involves a method of making a ceramic shellmold comprising repeatedly coating a fugitive pattern of an article tobe cast with a ceramic slurry layer and applying on the ceramic slurrylayer a refractory stucco to form a plurality of ceramic slurry layersand stucco layers on the pattern wherein at least one of the stuccolayers is formed by applying discontinuous stucco fibers followed byapplying a granular stucco particles on the discontinuous stucco fibers.

[0006] In a preferred embodiment of the invention, the granular stuccoparticles are applied on randomly oriented discontinuous stucco fibersto pack the discontinuous stucco fibers down on the slurry layerunderlying the discontinuous fibers. The granular stucco particlespreferably are applied on the discontinuous stucco fibers while theunderlying slurry layer is still wet such that a majority of the packeddown discontinuous stucco fibers stick to the slurry layer. The granularstucco particles preferably are applied on the randomly orienteddiscontinuous stucco fibers to form a stucco layer comprising a mat ofthe discontinuous stucco fibers and the granular stucco on and in themat.

[0007] In an illustrative embodiment offered to illustrate but not limitthe invention, the granular stucco particles are applied by raining thegranular stucco particles by gravity down on the discontinuous stuccofibers.

[0008] The present invention also provides a ceramic shell mold whereinat least one of the stucco layers comprises the discontinuous stuccofibers and the granular stucco particles.

[0009] Shell molds pursuant to the invention are advantageous to resistmold splitting during the pattern removal operation.

[0010] The present invention will become more readily apparent from thefollowing detailed description.

DESCRIPTION OF THE DRAWINGS

[0011]FIG. 1 is a schematic diagram of a method of making a investmentshell mold pursuant to an illustrative embodiment of the invention.

[0012]FIG. 2A is a schematic partial view of a shell mold wall on thepattern showing the randomly oriented discontinuous stucco fibersapplied on a still wet refractory slurry layer before the stucco fibersare packed down to form a generally two dimensional mat.

[0013]FIG. 2B is a schematic partial view of a shell mold wall showingthe randomly oriented discontinuous stucco fibers applied on a still wetrefractory slurry layer and after the stucco fibers are packed down toform a generally two dimensional mat.

[0014]FIG. 3 is a photograph of a sectioned shell mold made withoutgranular stucco particles on the discontinuous stucco fibers.

[0015]FIG. 4 is a photograph of a sectioned shell mold wall madepursuant to a preferred method of the invention.

[0016]FIG. 5 is a perspective view of a wedge shaped pattern used tomake a shell mold for pattern removal trials.

DETAILED DESCRIPTION OF THE INVENTION

[0017]FIG. 1 illustrates schematically a lost wax ceramic shell moldbuilding process to which the invention is applicable where a ceramicshell mold is formed by repeatedly coating a fugitive pattern of thearticle cast with refractory flour slurry (i.e. ceramic flour in aliquid binder) to provide a slurry layer, draining excess slurry,stuccoing the slurry layer with refractory stucco to provide a stuccolayer on the slurry layer until a desired shell mold wall thickness isbuilt up. The fugitive pattern can comprise a conventional wax,wax/polymer blends, polymeric or other fugitive materials molded orotherwise formed to the shape of the article to be cast as is well knownin the art. Such fugitive patterns are removable from the green shellmold invested thereabout using conventional pattern removal techniquessuch as melting, leaching and/or vaporizing the pattern therefrom.

[0018] In FIG. 1, the pattern P is dipped in the refractory flour (e.g.ceramic powder) slurry 11 held in a vessel 10, drained of excessrefractory slurry by temporary holding the pattern above the vessel 10for a predetermined time, and then is stuccoed at a stucco-applyingstation 12 while the refractory slurry layer is still wet. The pattern Ptypically is moved by a robot arm R. In practice of an illustrativeembodiment of the invention, the stucco-applying station 12 comprises afiber stucco-applying apparatus 12 a for applying discontinuous stuccofibers 14 to the pattern and a granular stucco-applying apparatus 12 bfor applying granular stucco particles 15 to form at least one of thestucco layers to comprise both discontinuous stucco fibers and granularstucco fibers pursuant to the invention. Other stucco layers maycomprise only granular stucco particles, which are applied to thepattern at only the granular stucco-applying apparatus 12 b. That is,the fiber stucco-applying apparatus 12 a is not used when the stuccolayer comprises only granular stucco particles. The stucco applyingapparatus 12 a, 12 b can comprise conventional stucco towers having abin 20 a, 20 b respectively, in which discontinuous stucco fibers andgranular stucco particles reside, respectively.

[0019] The pattern P with the stuccoed refractory slurry layer then isdried in air or in a conventional drying apparatus. After drying, thepattern P is subjected to similar dipping, draining, stuccoing anddrying operations until the desired shell mold wall thickness is builtup on the pattern. Drying of ceramic slurry/stucco layers is describedin U.S. Pat. Nos. 2,932,864; 4,114,285; and others as well as U.S. Ser.No. 09/690,144 of common assignee herewith.

[0020] Typically, in practicing the lost wax process, one or moreso-called prime coat (refractory slurry) layers and prime coat stuccolayers are applied to the pattern initially to provide a facecoat forcontacting the molten metal or alloy to be cast in the shell mold. Then,the facecoated pattern is subjected to repeated steps of slurry dipping,draining, stuccoing and drying steps to form back-up slurry layer/stuccolayers on the prime coat slurry layer(s) until the desired shell moldwall thickness is built-up. In general, the prime coat(s) employ(s) afiner refractory flour in the slurry than that present in the back-upslurries. The prime coat stucco similarly is a less coarse stucco thanthe back-up stucco. The prime coat slurry/stucco typically comprise arespective refractory material, such as a ceramic, to form a facecoatsuitably for contacting the molten metal or alloy being cast withoutadverse reaction therewith. The back-up slurry and back-up stucco cancomprise a refractory flour and refractory stucco which may be differentor the same as those used for the prime coat slurry/stucco. Therefractory flours/stuccoes used in the shell mold layers for castingnickel base and cobalt base superalloys typically comprise ceramicflours/stucco as described in U.S. Pat. Nos. 4,966,225, 5,335,717,5,975,188 and others, although refractory materials such as graphite,nitrides, carbides, and other materials may be used as described forexample in U.S. Pat. No. 5,297,615, the teachings of all of thesepatents being incorporated herein by reference.

[0021] The present invention involves forming at least one, preferably aplurality, of the stucco layers of the shell mold by applyingdiscontinuous stucco fibers followed by applying granular stuccoparticles on the discontinuous stucco fibers. For example, in anembodiment of the invention offered for purposes of illustration and notlimitation, the granular stucco particles 15, FIG. 1, are applied atapparatus 12 b on the discontinuous stucco fibers 14 previously appliedat apparatus 12 a, FIG. 1, to impact and pack the initially randomlyoriented discontinuous stucco fibers 14, FIG. 2A, down on the slurrylayer underlying the discontinuous fibers. The granular stucco particles15 preferably are applied on the discontinuous stucco fibers 14 whilethe underlying slurry layer is still wet such that a majority,preferably greater than 75% and more preferably 80-90%, of thediscontinuous stucco fibers 14 are packed down and stick to the slurrylayer as a result of impact by the falling granular stucco particles 15.The granular stucco particles preferably are applied on the randomlyoriented discontinuous stucco fibers to form a stucco layer comprising agenerally two dimensional mat of packed down discontinuous stucco fibers14 and the granular stucco particles on and in the mat, FIG. 2B, wherethe granular stucco particles are represented by dots in FIG. 2B.

[0022] The discontinuous stucco fibers can comprise silica, alumina, orother refractory materials suitable for the particular mold being formedand the particular casting parameters to be used. The stucco fibers arediscontinuous, relatively short fiber lengths having a length greaterthan fiber diameter. The stucco fibers preferably have lengths notexceeding ½ inch, typically in the range of ¼ to ⅜ inch, for purposes ofillustration only and fiber aspect ratios (length to diameter ratio) inthe range of 10 to 100, although the invention is not limited to theseranges. The granular stucco particles are characterized as having ablocky grain morphology and aspect ratio less than 2, a particle shapetypical of granular stuccoes used heretofore in the lost wax shell moldprocess and described in the above cited U.S. Patents. The granularstucco particles can comprise silica, alumina or other suitablerefractory stucco materials suitable for the particular mold beingformed and the particular casting parameters to be used. Thediscontinuous stucco fibers and the granular stucco particles comprisethe same or different refractory or ceramic material.

[0023] The stucco applying apparatus 12 a, 12 b can compriseconventional stucco towers having an elevated bin 20 a, 20 b,respectively, in which discontinuous stucco fibers 14 and granularstucco particles 15 reside, respectively. At apparatus 12 a, thediscontinuous stucco fibers 14 are released from the bin 20 a to fall orrain down by gravity on the wet refractory layer on the pattern P, whichis disposed a preselected distance (e.g. 20 to 60 inches) below the bin20 a and rotated so that the stucco fibers will impact and cover theentire surface area of the wet refractory slurry layer. Typically, thediscontinuous stucco fibers 14 are released from bin 20 a until stuccofibers are observed to fall off of (not stick to) the pattern as aresult of its being completely covered by previously released stuccofibers 14, although a predetermined amount of stucco fibers can bereleased over time. The discontinuous stucco fibers 14 stick to the wetslurry layer in a three dimensional array of randomly oriented fibers 14as illustrated in FIG. 2A. At apparatus 12 b, the granular stuccoparticles 15 are released from the bin 20 b to fall or rain down bygravity on the fiber-stuccoed, still wet refractory layer on the patternP disposed a preselected distance (e.g. 20-60 inches) below the bin 20 band rotated so that the granular stucco particles impact the stuccofibers and pack them down on the still wet refractory slurry layer.Typically, the granular stucco particles 15 are released from bin 20 buntil stucco particles are observed to fall off of (not stick to) thepattern as a result of its being completely covered by previouslyreleased granular stucco particles 15, although a predetermined amountof granular stucco particles can be released over time. Any conventionalstucco tower can be used in practicing the invention. A particularstucco tower which can be used to practice the invention is described inU.S. Ser. No. 09/690,144 filed Jul. 27, 2000, of common assigneeherewith, the teachings of which are incorporated herein by reference.

[0024] The fiber stucco or granular stucco particles can be applied tothe pattern by other means including spray coating, fluidized bedcoating or other techniques which provide sufficient energy to thestucco particles to pack down the stucco fibers to form a twodimensional mat type structure on the pattern.

[0025] The one or more stucco layers formed pursuant to the invention byapplying discontinuous stucco fibers 14 followed by applying a granularstucco particles 15 on the randomly oriented discontinuous stucco fiberscan preferably comprise intermediate stucco layers of shell mold wall25, although the invention is not limited in this regard. For example,for purposes of illustration and not limitation, the stucco layercomprising the discontinuous stucco fibers 14 and granular stuccoparticles 15 can comprise the 4th, 5th, 6th, etc. intermediate stuccolayers of the shell mold wall as it is being built-up layer-by-layer.

[0026] The stucco layer(s) formed pursuant to the invention by applyinggranular stucco particles 15 on the randomly oriented discontinuousstucco fibers 14 of FIG. 2B exhibit less porosity for a given shell moldwall thickness and less fiber-bridging (where fibers bridge across oneanother creating a void) than a stucco layer comprising onlydiscontinuous stucco fibers. Application of the granular stucco fibersrearranges the randomly oriented discontinuous stucco fibers to providea higher fiber packing density with some granular stucco particles 15filling spaces between the stucco fibers 14 and a more dense shell moldwall 25, compare FIGS. 3 and 4.

[0027] Shell molds pursuant to the invention exhibit greater strength intension and greater toughness (resistance to crack propagation) thanshell molds without one or more of the composite slurry layers(comprising the discontinuous stucco fibers 14 and granular stuccoparticles 15) and are advantageous to resist mold splitting during thepattern removal operation.

[0028] The following Examples are offered to further illustrate theinvention without limiting it.

EXAMPLES Example 1

[0029] Shell molds were made by the lost wax process described in U.S.Pat. No. 4,966,225 by applying to identical wax patterns ceramic slurrylayers/stucco layers as shown in Table I below: TABLE I Sample B SampleA (w/o fiber) (w/o stucco packing) Sample C (w/ stucco packing) SlurryStucco Slurry Stucco Slurry Stucco 1^(st) A −120 Alumina A −120 AluminaA −120 Alumina 2^(nd) B/C  −90 Alumina B/C  −90 Alumina B/C  −90 Alumina3^(rd) B/C 28 × 48 Tab Alumina B/C 28 × 48 Tab Alumina B/C 28 × 48 TabAlumina 4^(th) B/C 14 × 28 Tab Alumina B/C ¼″ Q Fiber B/C ¼″ Q Fiber +14 × 28 Tab Alumina 5^(th) C 14 × 28 Tab Alumina C ¼″ Q Fiber C ¼″ QFiber + 14 × 28 Tab Alumina 6^(th) C 14 × 28 Tab Alumina C ¼″ Q Fiber C¼″ Q Fiber + 14 × 28 Tab Alumina 7^(th) C 14 × 28 Tab Alumina C 14 × 28Tab Alumina C 14 × 28 Tab Alumina 8^(th) C 14 × 28 Tab Alumina C 14 × 28Tab Alumina C 14 × 28 Tab Alumina 9^(th) C C C

[0030] It is apparent from the Table that the ceramic slurries andstuccoes of the 1st (facecoat), 2nd, 3rd, 7th, and 8th layers were thesame. Slurry A comprised an alumina-based slurry using a 12 nm sizecolloidal silica binder liquid (LUDOX HS30 binder from Grace ChemicalsCorp.). Slurries B, C and D each comprised a zircon-based slurry usingthe 12 nm size colloidal silica binder liquid. Dips using two slurriesare designated B/C (and B/D in later examples) and represent the wellknown practice of initially dipping in a low viscosity slurry followedby dipping in a standard, higher viscosity slurry. The stucco for the1st stucco layer was −120 mesh fused alumina granular stucco (−120 meshmeaning less than 120 mesh particles). The stucco for the 2nd stuccolayer was −90 mesh fused alumina granular stucco. The stucco for the 3rdstucco layer was 28×48 mesh tabular alumina granular stucco where thestucco particles have a particle size less than 28 mesh and greater than48 mesh. The stucco for the remaining layers was 14×28 mesh tabularalumina granular stucco. As is apparent, the ceramic slurry used for the4th, 5th, and 6th layers also were the same. However, 4th, 5th, and 6thstucco layers were different in that in making mold sample A, the 4th,5th, and 6th stucco layers comprised only 14×28 tabular alumina, whereinin making mold sample B, the 4th, 5th, and 6th stucco layers comprisedonly ¼ inch long chopped (discontinuous) “Q” fibers. In making moldsample C, the 4th, 5th, and 6th stucco layers comprised the ¼ inch longchopped “Q” fibers followed by application of granular 14×28 stuccoparticles for fiber stucco packing pursuant to the invention.

[0031] The discontinuous chopped “Q” fibers comprised silica and had adiameter in the range of 9 to 14 microns. The unchopped “Q” fibers(Quartzel silica) are available from Saint-Gobain Quartz, 1600 W. LeeSt., Louisville, Ky. The “Q” fibers were chopped by OMNIA LLC, Raleigh,N.C. The 14×28 granular stucco particles comprised grains having aparticle size of less than 28 mesh and greater than 48 mesh andcomprised tabular alumina. The 14×28 alumina granular stucco particlesare available from Alcoa Alumina and Chemicals, Bauxite, Ark. Mesh sizesare with respect to U.S. standard screen system. In making the moldsamples, both the “Q” stucco fibers and 14×28 alumina granular stuccoparticles were applied to the pattern by free fall from 5 feet above thepattern as each mold sample was being built-up.

[0032]FIGS. 3 and 4 are photographs of the built-up wall of shell moldsample B and C, respectively. The difference between shell mold B and Cis dramatic in that the mold sample C exhibits less porosity for a givenshell mold wall thickness and less fiber-bridging (where fibers bridgeacross one another creating a void). Random orientation of the stuccofibers and fiber-bridging are apparent in sample B in FIG. 3, both ofwhich increase wall porosity and the number of void defects in the moldwall, reducing mold strength. For example, the projections PJ on theouter mold sample surface (left hand side of FIG. 3) are discontinuousstucco “Q” fibers oriented outwardly and transversely of the plane ofthe sample wall. Several large voids L are apparent and associated withthe discontinuous stucco fibers. In FIG. 4, application of the granularstucco particles has rearranged the discontinuous stucco fibers toprovide a higher fiber packing density with granular stucco particlesfilling spaces between the stucco fibers to produce a more dense andstronger shell mold wall.

[0033] Mechanical properties were determined for mold samples A, B and Cand are set forth in Table II below: TABLE II Sample A Sample B Sample CMOR (Psi) 830 657 900 EBP 0.059 0.253 0.307 Shell Porosity (%) 20.4 28.523.6

[0034] As the results show, mold sample C retained similar strength andporosity as sample A with improved EBP (EBP is energy to break pointexpressed in units of lbf-in). Sample B, however, became weaker and moreporous due to fiber bridging. The overall mechanical properties ofsample C pursuant to the invention are improved significantly, so thatcracking probability of the shell mold should be reduced.

Example 2

[0035] This example describes how shell mold performance during patternremoval can be improved by practice of the invention. A wedge shaped waxpattern, FIG. 5, was used to test the probability of shell moldcracking. This wedge shaped pattern can often cause shell mold crackingalong the edges of the wedge.

[0036] Test wedge shaped shell molds were made as shown in Tables IIIbelow. Slurry A and B were equivalent to slurry A and B in Example 1.Slurry D was similar to slurry C in Example 1 with a higher organicbinder content. Some wedge shaped molds A1, B1, C1 were made with nofiber reinforcement, and the other set A2, B2, C2 was made with “Q”fiber reinforcement followed by application on the fibers of 14×28tabular alumina stucco (designated 14-28 in Table III) pursuant to theinvention and as described in Example 1. After steam dewaxing operation,each wedge shaped mold was inspected, and the probability of crackingwas calculated based on percentage of cracked wedge shaped molds out ofthe total shell molds. TABLE III Wedge Specimen A1 (9-layer shell) WedgeSpecimen A2 (9-layer shell) Dip Slurry Stucco size Dip Slurry Stucco 1stA 120 1st A 120 2nd B/D  90 2nd B/D  90 3rd B/D 28-48 3rd B/D 28-48 4thD 14-28 4th D ¼″ Q-fiber, 14-28 5th D 14-28 5th D 14-28 6th D 14-28 6thD 14-28 7th D 14-28 7th D ¼″ Q-fiber, 14-28 8th D 14-28 8th D 14-28 9thD D Wedge Specimen B1 (8-layer shell) Wedge Specimen B2 (8-layer shell)Dip Slurry Stucco size Dip Slurry Stucco 1st A 120 1st A 120 2nd B/D  902nd B/D  90 3rd B/D 28-48 3rd B/D 28-48 4th D 14-28 4th D ¼″ Q-fiber,14-28 5th D 14-28 5th D 14-28 6th D 14-28 6th D ¼″ Q-fiber, 14-28 7th D14-28 7th D 14-28 8th D 8th D Wedge Specimen C1 (7-layer shell) WedgeSpecimen C2 (7-layer shell) Dip Slurry Stucco size Dip Slurry Stucco 1stA 120 1st A 120 2nd B/D  90 2nd B/D  90 3rd B/D 28-48 3rd B/D 28-48 4thD 14-28 4th D ¼″ Q-fiber, 14-28 5th D 14-28 5th D ¼″ Q-fiber, 14-28 6thD 14-28 6th D 14-28 7th D 7th D

[0037] wherein “28-48” and “14-28” in Tables III correspond to 28×48 and14×28 mesh size granular stucco in Table I.

[0038] Wedge shaped shell mold specimens A1-C2 were placed in a steamautoclave to remove the wax pattern. After steam dewaxing operation,each wedge shaped mold was inspected, and the probability of cracking(prob. cracking) was calculated based on percentage of cracked wedgeshaped molds out of the total shell molds and listed in Table IV below.TABLE IV Wedge Specimen prob. Cracking (%) A1 100 A2 0 B1 100 B2 0 C1100 C2 0

[0039] Under steam autoclave dewaxing conditions, the Q fiber-reinforcedshell molds A2, B2, C2 showed no cracking for any of the molds tested,whereas all of the standard (non-Q fiber reinforced) shell molds A1, B1,C1 were cracked. Mold sample C2 with only 7 layers including 2 Q fiberlayers had no cracks as compared with the thicker 9-layer mold sample A1with 100% cracking probability. Dewaxing performance of the shell moldsproduced pursuant to the invention is substantially improved.

[0040] The above sample shell molds also were subjected to furnacedewaxing where a furnace was first heated to 1600 degrees F. Then, thewedge shaped sample shell molds were pushed into the furnace to removethe wax pattern and then inspected after removal from the furnace. Theprobability of cracking (prob. cracking) for each sample shell mold wascalculated and listed in Table V below:. TABLE V Wedge Specimen prob.Cracking (%) A1 0 A2 0 B1 100 B2 0 C1 100 C2 25

[0041] Under the flash dewax conditions of the heated furnace, theexperimental results demonstrated significant reductions of shell moldcracking by practice of the invention. For example, the 8-layer shellmold with Q fiber reinforcement (sample B2) had no cracking, while thestandard shell molds without Q fiber reinforcement (sample B1) were allcracked.

[0042] Although the present invention has been described with respect tocertain specific illustrative embodiments thereof, it is not so limitedand can be modified and changed within the spirit and scope of theinvention as set forth in the appended claims.

We claim:
 1. A method of making a ceramic shell mold, comprisingrepeatedly coating a fugitive pattern of an article to be cast with aceramic slurry layer and applying on the ceramic slurry layer arefractory stucco to form a plurality of ceramic slurry layers andstucco layers on the pattern wherein at least one of the stucco layersis formed by applying discontinuous stucco fibers followed by applying agranular stucco particles on the discontinuous stucco fibers.
 2. Themethod of claim 1 wherein the granular stucco particles are applied onrandomly oriented discontinuous stucco fibers to pack the discontinuousstucco fibers down on the slurry layer underlying the discontinuousfibers.
 3. The method of claim 2 wherein the granular stucco particlesare applied on the randomly oriented discontinuous stucco fibers whilethe underlying slurry layer is still wet such that a majority of thepacked down discontinuous stucco fibers stick to the slurry layer. 4.The method of claim 2 wherein the granular stucco particles are appliedon the randomly oriented discontinuous stucco fibers to form a stuccolayer comprising packed down discontinuous stucco fibers and thegranular stucco.
 5. The method of claim 4 wherein some of the granularstucco particles fill spaces between the packed down discontinuousstucco fibers.
 6. The method of claim 1 wherein the granular stuccoparticles are applied by raining the granular stucco particles bygravity down on the discontinuous stucco fibers.
 7. The method of claim6 wherein the discontinuous stucco fibers and the granular stuccoparticles comprise the same or different ceramic material.
 8. A ceramicshell mold, comprising a plurality of ceramic flour layers andrefractory stucco layers wherein at least one of the stucco layerscomprises both discontinuous stucco fibers and granular stuccoparticles.
 9. The shell mold of claim 8 wherein said at least one stuccolayer comprises packed down discontinuous stucco fibers with thegranular stucco on and between the fibers.
 10. The shell mold of claim 9wherein some of the granular stucco particles occupy spaces between thepacked down discontinuous stucco fibers.
 11. The shell mold of claim 8wherein the discontinuous stucco fibers and the granular stuccoparticles comprise the same or different ceramic material.